1. Field of Invention
The invention relates to a lens and a light-emitting diode (LED) using the lens for illumination systems and, in particular, to an LED with homogeneous illumination.
2. Related Art
A traditional LED 10 is shown in FIG. 1A. It is formed by packing an LED chip 12 in a lens 11. The most commonly seen embodiment of the lens is illustrated in the drawing. The lens 11 is a hemisphere to converge the light emitted by the LED chip 12. The radiation pattern (directions and energy distribution of the emitted light) has to be restricted in the vicinity of the optical axis 13. Since the energy is more concentrated, one has to rely on the use of a light-guiding plate 14 to improve the homogeneity in the case of a surface plane light source. However, if the LED's 10 are directly distributed under the light-guiding plate, it requires a fair amount of LED's 10 because each one of them has a very small illuminating area. Therefore, one usually adopts the side-illuminating structure to achieve the required homogeneity of a surface plane light source, as shown in FIG. 1B. However, since the LED's 10 can provide the optical energy only at the edge of the light-guiding plate 14, the distance between the central part and the adjacent light sources becomes larger when the area of the surface plane light source is big. Therefore, the optical energy transmitted to the central part by virtue of total reflections become insufficient as the area enlarges. In other words, there is a certain limit for the area of the surface plane light source.
To overcome this problem, U.S. Pat. No. 6,679,621 uses a special lens shape to totally reflect the light coming out of the top surface of the LED and to deflect the light coming out of its side surfaces. All beams are emitted in parallel but orthogonal to the optical axis. Therefore, in the construction of a planar light surface, the LED's can be disposed in the middle of the light-guiding plate, instead of being limited to its edges. As the area of the surface plane light source gets larger, there will not be any problem of insufficient optical energy on the light-guiding plate due to the far distances to the light source. Nevertheless, the design relies on the total reflection and refraction of light. The overall light-emitting efficiency is lowered due to the attenuation of multiple reflections and refractions inside the light source system.